For all season tires having a good running performance on snow-covered roads (i.e., on-snow performance) improved with maintaining a high speed running performance, it is important to enhance a traction performance on snow-covered roads. For this purpose, zigzag grooves have been generally used as circumferential main grooves extending in a circumferential direction of tire. However, as shown in FIG. 11(A), external corner portions “g1a” projecting toward a groove width center side in zigzag grooves “g1” are low in rigidity and, therefore, amount of slip of these portions with a road is large. Therefore, there is a problem that such an uneven wear that the external corner portions “g1a” are worn away early generates to result in lowering of dry grip performance. In particular, since a large load is applied during cornering to an outer region of a tread surface which is located on an outer side of a vehicle, circumferential main grooves disposed in the outer region cause uneven wear more markedly. Moreover, since an influence of uneven wear on cornering force is also great, lowering of the dry grip performance is further accelerated.
On the other hand, it is also known to use, as a circumferential main groove, a straight groove which extends straight in the circumferential direction. However, since no traction force is obtained by the straight groove on snow-covered roads, the on-snow performance cannot be enhanced and, therefore, the straight groove is disadvantageous to all season tires. Furthermore, although the straight groove causes less uneven wear, corner portions “q” at which groove wall surfaces “gs” intersect with a tread surface “ts” as shown in FIG. 11(B) are low in rigidity. Therefore, when a straight groove is disposed in the outer region mentioned above, uneven wear resistance is still on an unsatisfactory level such that the corner portions “q” are worn away early by a large load generating at the time of cornering at a high speed.